I. Field of the Invention
The present invention relates to novel reversible gecko-inspired adhesives, and a method of producing novel reversible gecko-inspired adhesives using shear to incorporate tilt into the microstructure of novel reversible gecko-inspired adhesives.
II. General Background
The superior adhesive properties of geckos, which allow them to repeatedly attach strongly and detach easily from a variety of surfaces, have inspired the fabrication of synthetic dry adhesives. However, an adhesive material that exhibits all the characteristics of gecko adhesion still remains elusive. The superior adhesive properties of geckos are attributed to the complex, hierarchical topography of fine tilted and curved columnar structures (setae) each bearing multiple trapezoidal-shaped terminal pads (spatulae) that make up the adhesive arrays on gecko toes. Natural gecko setal arrays show strong adhesion and friction forces originating from van der Waals interactions when sheared in the “gripping direction” towards the axis of tilt, but show no adhesion and smaller friction when sheared in the opposite “releasing direction.” This mechanically-controllable adhesion anisotropy allows for easy and rapid locomotion on vertical and inverted surfaces.
Synthetic adhesives based on a variety of materials and fabrication schemes have been developed to mimic gecko adhesion, yet a true mimic still remains elusive because of limitations in current nanofabrication techniques. The Dhinojwala group used chemical vapor deposition to create gecko-inspired adhesive tape based on high-density carbon nanotubes. The Sitti group fabricated by soft lithography approaches slanted polyurethane-based microstructures that showed anisotropic adhesion. The del Campo group used a combination of multi-step photolithography and soft lithography to generate hierarchical structures. Jeong et al. fabricated micro/nanoscale hierarchical polymeric hairs using a two-step UV-assisted capillary molding technique. The Cutkosky group used microfabrication techniques to develop single level wedge-shaped dry adhesives with anisotropic properties, and the Israelachvili group used photolithography and molding techniques to create microflaps composed of polydimethylsiloxane, which also showed anisotropic tribological properties.
Although anisotropic soft polymer structures (elastic modulus E˜0.1-1 MPa) had been fabricated previously, in 2008, the Fearing group was the first to fabricate anisotropic gecko-inspired dry adhesives from a hard polymer, polypropylene (E˜1.4 GPa). The Turner group reported that improved adhesion could be obtained with the integration of nano and microstructures due to enhanced structural compliance, and the Bhushan group provided a theoretical analysis on a multi-level hierarchical structure showing adhesion enhancement to rough surfaces.
While a major focus in fabricating gecko-like adhesives has been on the geometrical design of the structures, recent work has shown that chemistry—i.e., surface functionality and presence of lipids—should also be considered in their future designs. The above synthetic dry adhesives either do not show anisotropic adhesion or rely on multi-step, expensive fabrication schemes, which are not easily scalable for hierarchical structures. There is thus a need for an easy, scalable fabrication scheme for dry, reversible adhesives that exhibit anisotropic adhesion. The present invention can be shown to have numerous advantages over conventional adhesives such as tapes that rely on tacky filaments and over other synthetic dry adhesives.
While certain novel features of this invention shown and described below are pointed out in the claims, the invention is not intended to be limited to the details specified, since a person of ordinary skill in the relevant art will understand that various omissions, modifications, substitutions and changes in the forms and details of the invention illustrated and in its operation may be made without departing in any way from the spirit of the present invention.